Plant and human pathogens utilize type III secretion systems to inject effector proteins into host cells. In plants, effectors can elicit a hypersensitive response causing rapid cell death to contain bacterial infection, adjust plant defenses, and modulate host metabolism to be beneficial for pathogenesis. Prior work on the role of effectors in bacterial fitness utilized models that bypassed initial host-pathogen interactions on leaf surfaces. This study will examine the role of effectors in the fitness of pathogens on leaf surfaces. Effector expression of Pseudomonas syringae during the epiphytic association of P. syringae on tobacco plants will be examined under varying growth conditions. Effector deletion mutants will be examined to determine whether particular effectors are required for epiphytic survival of P. syringae. The effect of known defense mechanisms on epiphytic populations will be studied using tobacco with silenced defense genes. Understanding the role of effectors in epiphytic growth of bacteria will better allow us to understand plant pathogenesis and devise novel control strategies. [unreadable] [unreadable] [unreadable]